The present invention is due to Mr. Guy AUBERT, Director of the Service National des Champs Intenses, and its object is a magnet, of a generally substantially spherical shape, with equatorial access, to produce a uniform induction field. It finds application particularly in the medical field where magnets are used in experiments of imaging by nuclear magnetic resonance. It can also find application in all fields where such magnetic induction fields are required.
2. Discussion of the Background
In the field of imaging by magnetic resonance, it is necessary to place the objects to be imaged, the patients in medical field, in a high magnetic induction field (usually of 0.1 to 1.5 Tesla) which is homogenous and uniform (with a few parts per million of variation) in a large volume of interest (commonly a sphere of 50 cm diameter). Several classes of magnetic field generators have been developed until now. The main ones are: superconductive magnets, so-called resistive magnets and permanent magnets. Permanent magnets have many advantages. In particular, they require no energy supply to produce the field. They therefore do not run the risk of drift in their field value due to a drift of their supply, or possibly of the system for discharging the dissipated heat. They therefore call for no cooling means, in particular with sophisticated regulation techniques for the flow of cryogenic fluids. Their working temperature is easily stabilized. They are furthermore particularly suited to the making of structures or systems producing a transversal main field, namely a field perpendicular to a direction in which objects, patients, are introduced into the magnet. This arrangement is highly favorable to the making of receiver antennas of highly uniform and high gain resonance signals. A major drawback of permanent magnets is located, however, at the level of their industrial-scale manufacture.
Permanent magnet structures producing a transversal, uniform magnetic field in a relatively big volume have been described in the state of the art. In particular, in an international patent application No. WO 84/01226 filed on 23rd Sept. 1983 and published on 29th Mar. 1984, D. LEE et al. have described a magnet of this type. In it, a cylindrical structure (theoretically of infinite length) is approximated by a stacking of a certain number of annular sections each provided with a certain number of magnetized blocks. The blocks are distributed on the rim of the rings in a polygonal architecture which reproduces, as far as possible, the circular appearance of a theoretical cylinder. To produce a field transversal to the axis of the cylinder, the magnetization in each of the blocks is constant as regards modulus and is oriented, with respect to the direction of the induction field to be produced, with an angle equal to or twice that measured by the positioning angle of the block in question. The blocks described are, in a preferred way, prismatic volumes with a trapezoidal section.
The result of the distribution of magnetization thus proposed is that the magnetization of certain blocks has to be oriented, with respect to this block, in a direction which is parallel with none of the sides of the trapezoidal section. The making of magnetic blocks of this type therefore necessitates the use, industrially, of special magnetizers. While this use, albeit costlier than the use of standard magnetizers, is still possible, the same is not the case for the forming of the blocks. In effect, the distribution of the magnetization imposed in the cylinder creates a demagnetizing excitation, the orientation of which is rarely parallel, in each block, to that of the magnetization. This implies, for the fabrication, the choice of so-called anisotropic magnetic materials. Now, anisotropic magnetic materials which, besides, have the best magnetic properties, have the drawback of being hard to machine in directions that are oblique with respect to the direction of their anisotropy. The above-mentioned patent application indicates, especially in its FIG. 5 and in the associated text, that the making of the blocks can be obtained by a stacking of elementary bricks. However, it is clear that elementary bricks, of parallelepiped shape, have a favored direction of magnetization which is parallel to one side of the parallelepiped. Hence the fact remains that it is difficult, on the one hand, to cut the bricks obliquely with respect to the sides of this parallelepiped or, on the other hand, to efficiently magnetize the blocks formed in directions that are oblique with respect to the sides of these parallelepipeds. Consequently, in the structure presented, certain blocks, those in the alignment of the bisectors of the four quadrants, cannot easily be magnetized. The distribution of the magnetization in this magnet further leads to a corresponding distribution of the demagnetizing excitation. This is such therein that, at places, it may be sufficient to substantially diminish the magnetization. Consequently, the theoretically calculated magnet cannot be made and the performances of the real magnet are quite removed from the ideal.
A magnet of this type further has other drawbacks. In particular, there is no equipment entrance possible into the interior of the zone of interest apart from the axial entrance. Finally, above all, cylindrical magnets present a drawback related to the cyldrical shape itself. With a cylindrical shape, it is necessary for the magnetic blocks located at the ends of a cylinder to be big since, in brief, they represent and replace the extension to infinity of the two ends of the cylinder. Now, in view of their distance, they contribute with little efficiency to the intensity of the induced field: they have, above all, the effect of improving its homogeneity. Their high mass and, therefore, their weight related to the problems of the cost of the magnetic materials is a brake on the use of permanent magnets.